Biaxially oriented polypropylene films are typically used for packaging, decorative, and label applications and often performs multiple functions. In a lamination, they provide printability, transparent or matte appearance, or slip properties. The films sometimes provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties. The films sometimes provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or by laminating.
However, in recent years, interest in “greener” packaging has been strongly developing. Packaging materials based on biologically derived polymers are increasing due to concerns with renewable resources, raw materials, and greenhouse gas generation. Bio-based polymers are believed—once fully scaled-up—to help reduce reliance on petroleum, reduce production of greenhouse gases, and can be biodegradable or compostable as well. Bio-based polymers such as polylactic acid (PLA)—which is currently derived from corn starch (but can be derived from other plant sugars) and thus, can be considered to be derived from a renewable resource—is one of the more popular and commercially available materials available for packaging film applications. Other bio-based polymers such as polyhydroxyalkanoates (PHA) and particularly polyhydroxybutyrate (PHB), are also of high interest, and are starting to become commercially available at the time of this writing.
Typically, biaxially oriented polylactic acid (BOPLA) films are transparent with a high clarity and high gloss. In some packaging applications, this is very desirable for printing graphics with high visual appeal—or “pop”—and to provide high resolution of the desired images. Polylactic acid (PLA), being a polar polymer, naturally has a high surface energy that helps enable good wettability of the types of printing inks and colors used in packaging. In addition, multi-layer BOPLA films can be formed via coextrusion of several melt streams of PLA-based resins. One example could be a two-layer coextruded film structure in which a base or core layer including a crystalline PLA and a thinner “skin” layer including amorphous PLA is coextruded upon one side of the core layer and then biaxially oriented into a film. The amorphous PLA layer is often used to provide heat sealability to the film as it is less crystalline and generally has a lower melting point than the higher crystallinity core layer.
For such a bio-based polymer to be fit-for-use for many snack food-packaging applications, it is desirable that the bio-based polymer film match as many of the attributes possible that BOPP is well known for, such as heat sealability, printability, controlled COF, metallizability, gas barrier, etc. Since BOPLA film tends to have poorer thermal dimensional shrinkage properties in the machine and transverse direction orientations than BOPP film does, a BOPLA lamination may not perform as well on a packaging machine optimized for BOPP laminations. It has been noted that bag-making on a packaging machine designed for BOPP laminations has certain expectations for heat seal setpoints and bag-making speeds. It has also been found that replacing an all-BOPP lamination with an all-BOPLA lamination can—in some cases—result in distortion of the bag around the heat-sealed areas, namely the end seals of the bag and the back seal of the bag. Although the typical amorphous PLA used as a heat seal layer in BOPLA films generally has a lower melting point and lower seal initiation temperature than conventional propylene-based heat seal resins (e.g. copolymers of propylene, ethylene, and/or butylene), in a typical packaging machine designed for BOPP laminations, the heated sealing bars are generally too hot for BOPLA laminations causing distortion in the sealed areas.
This is aesthetically unacceptable to many food packaging companies. Solutions to circumvent the distortion issue with BOPLA laminations involve reducing the heat sealing bar temperature setpoint or lowering the bag-making speed of the packaging machine. However, it has been found that temperature control and consistency of the heat sealing bars are highly variable and insufficient to reliably control the distortion problems, especially across a large fleet of packaging machines that may include different models and designs; and lowering the bag-making speed is generally unacceptable due to unit cost issues of the bagged product. One solution could be to improve the thermal stability of BOPLA films; another solution could be to improve the seal range of BOPLA films by lowering the seal initiation temperature of the amorphous PLA sealant layer such that high bag-making speeds can be maintained with lower setpoint temperatures on the sealing bars.
Thus, it is often desirable to have heat sealable films and laminations with a low sealing initiation temperature. Lower sealing temperatures can improve productivity of packaging machines and help lower overall product costs. By being able to seal packages at lower temperatures, the packager can reduce energy usage and costs. Further, by having packages made with low seal initiating materials, productivity can be improved by the ability to run packaging machines at a higher speed or higher number of packages per minute. This is due to a reduced dwell time required for sufficient thermal transfer of heat energy from the packaging machine's sealing bars to the sealant resin.
U.S. Pat. No. 7,128,969 describes a film composed of a base layer of PLA with a minority component of a thermoplastic or polyolefin such as polypropylene or polyethylene, typically less than 1% by weight of the base layer. Such a formulation is particularly suitable for thermoforming or biaxial stretching by means of pneumatic drawing or other mechanical forming. However, the invention is not suitable for high transverse orientation rates in excess of 6 TDX; the highest TDX cited in the examples is 5.5. Such polyolefins typically have a higher melting point that amorphous PLA and in addition, are petroleum-based and are non-compostable or degradable.
U.S. Pat. No. 7,368,160 describes a multi-layer coextruded biaxially oriented PLA film which uses an amorphous PLA resin that is heat sealable. However, there is no teaching for further improving the heat sealing properties or characteristics of such a film.
U.S. Pat. No. 6,649,732 describes a single layer extruded cast or oriented PLA film coated with a gas barrier material such as silicon oxide. The patent discloses that the film may be coextruded with a second resin but is silent on the nature of the second resin and does not indicate its improvement of heat-sealing properties.
U.S. Pat. No. 6,713,175 describes a single-layer biaxially oriented PLA film that is heat sealable using a PLA resin of a certain storage elastic modulus at 120° C. of 100-230 MPa. However, the patent is silent on improving further the heat seal properties of such a film. Moreover, the use of a single-layer heat sealable film can give rise to other issues in terms of film handling, processing, and sticking to equipment parts.
U.S. Pat. No. 6,153,276 describes a heat sealable PLA-based polymer laminate having an amorphous PLA layer and a crystalline PLA base layer. However, the patent is silent on methods to improve further the heat sealable properties of amorphous PLA.
U.S. Pat. No. 7,390,558 describes multi-layer biaxially oriented PLA film composed of two layers of varying blends of crystalline and amorphous PLA wherein one layer has the amorphous PLA as a majority component and the second layer has the amorphous PLA as a minority component. The heat sealable layer would presumably be the layer containing a majority of amorphous PLA. It would be expected that the heat seal initiation temperature of a blended layer of amorphous PLA and crystalline PLA would be worse than that of a wholly amorphous PLA layer.
U.S. patent application Ser. No. 12/333,047 describes a method to produce biaxially oriented PLA films at high transverse orientation rates using a specific processing aid such as an ethylene-acrylate copolymer. This reference is incorporated in its entirety in this application.
The inventors seek to address the above issues of making improved heat sealable biaxially oriented PLA films.